Piezoid materials, which include macrocrystalline substances such as quartz as well as composites of microcrystalline substances, develop a polarized electrical potential between their opposing surfaces when subjected to mechanical force. The strength of the electric potential developed between the opposing surfaces of the piezoid material is proportional to the strength of the mechanical force applied to the piezoid material. Because piezoid materials are highly sensitive to minute changes in pressure or force to which they are subjected, piezoid materials have been used in phonograph needles, microphones and other such force transducing devices. For example, U.S. Pat. No. 2,898,477 to Hoesterey describes a crystal microphone which translates sound into an electrical signal by exploiting the change in the magnitude of the electric potential developed between the faces of a piezoid material when subjected to mechanical forces in the form of acoustic waves.
In the device described by Hoesterey and other similar devices, the piezoid material, in the form of a symmetrical, thin quartz plate, is utilized as a dynamic force transducer, with the piezoid material employed as the dielectric material in a capacitor-type structure. In such a structure, conductive plates are applied to opposing surfaces of the piezoid material and, as the magnitude of the electric polarization charges between the opposing surfaces of the piezoid material changes in response to applied mechanical forces, unbound charged particles in the conductive plates respond to counterbalance the magnitude of the electric polarization between the opposing faces of the piezoid material. The reaction of the unbound charged particles in the conductive plates translates into an electrical signal representative of the acoustic waves to which the piezoid material is subjected.
As is the case with any capacitor-type structure, the output of the signal measured between the conductive plates decays to zero over time once sufficient unbound charged particles accumulate in the conductive plates to balance the electric potential across the dielectric, which in this case is a piezoid material. Accordingly, while such dynamic transducer devices are useful for measuring fluctuations in applied force over time, such devices are not useful for measuring a static force which does not change over time.
It is important to note that in force transducers using a piezoid material as the dielectric of a capacitor, even though the electric potential between the opposing faces of the piezoid material is counter-balanced by unbound charged particles in the conductive plates, causing the resulting signal to drop to zero, the potential developed across the piezoid material in response to a mechanical force remains constant. A piezoid material is an electret, and will retain the polarization of its dipolar electric field as long as an external, non-isotopic force is applied. Accordingly, a piezoid material has a measurable response to a constant, applied mechanical force, and this response can be used to determine the strength of the applied force. This is shown in devices such as those disclosed in U.S. Pat. No. 3,761,784 to Jund and U.S. Pat. No. 4,791,471 to Onodera et al. In each of those references, a piezoid material is physically disposed to act as the gate of a transistor. As a result, when the piezoid material is subjected to mechanical force, the electric potential between opposing surfaces of the piezoid material changes, and the resultant effective surface charge at the gate of the transistor changes, controlling the conductance of the channel of the transistor. Accordingly, the response of the transistor is proportional to the mechanical force applied to the piezoid material which acts as the gate to the transistor.
In these references, as well as in U.S. Pat. No. 4,378,510 to Bennett, a specially doped semiconductor region was directly mounted on the piezoid material. As shown in Bennett, N-type and P-type regions were created to create the transistor channel. However, the necessity of having both N type and P type regions creates some complexity in the manufacture of such devices.
It is against this background that even further significant improvements have evolved per the present invention in the field of measuring mechanical forces with piezoid materials.